ProSoft Technology MVI56E-MNETR - Manual (Page 107)

Reference MVI56E-MNETR ♦ ControlLogix Platform
User Manual Modbus TCP/IP Interface Module with Reduced Data Block
ProSoft Technology, Inc.
Page 107 of 153
August 31, 2009
The Read Block ID is an index value used to determine the location of where the
data will be placed in the ControlLogix processor controller tag array of module
read data. Each transfer can move up to 40 words (block offsets 1 to 40) of data.
In addition to moving user data, the block also contains status data for the
module.
The Write Block ID associated with the block requests data from the ControlLogix
processor. Under normal program operation, the module sequentially sends read
blocks and requests write blocks.
For example, if the application uses three read and two write blocks, the
sequence will be as follows:
R1W1
→R2W2→R3W1→R1W2→R2W1→R3W2→R1W1→
This sequence will continue until interrupted by other write block numbers sent by
the controller or by a command request from a node on the Modbus network or
operator control through the module’s Configuration/Debug port.
Write Block
These blocks of data transfer information from the ControlLogix processor to the
module. The following table describes the structure of the output image.
Offset Description Length
0 Write Block ID 1
1 to 40 Write Data 40
41 Spare 1
The Write Block ID is an index value used to determine the location in the
module’s database where the data will be placed. Each transfer can move up to
40 words (block offsets 1 to 40) of data.
Initialize Output Data
When the module performs a restart operation, it will request blocks of output
data from the processor to initialize the module’s output data (Read Data Area).
Use the Initialize Output Data parameter in the configuration file to bring the
module to a known state after a restart operation. The following table describes
the structure of the request block.
Offset Description Length
0 1000 to 1124 1
1 to 40 Spare 40
41 1000 to 1124 1
The block number in word 20 of the block determines the data set of up to 40
output words to transfer from the processor. Ladder logic in the processor must
recognize these blocks and place the correct information in the output image to
be returned to the module.